The possibility of using liquid-phase instead of vapor-phase oxidation for the preparation of benzene carboxylic acids was first indicated by the disclosure in U.S. Pat. No. 2,245,528 of the catalysis provided by transition of variable valence metals, especially cobalt, in a liquid phase of saturated lower aliphatic acid at temperatures from about 100.degree. C. to about 320.degree. C. and pressures to maintain the liquid phase of the aliphatic acid. Such catalysis, according to said patent, was advantageously promoted by the use of a ketone such as methylethyl ketone or aldehyde such as acetaldehyde. Unfortunately such aldehyde or ketone promoted variable valence metal catalysis was useful only for converting mono-, di- and tri-methylbenzenes to their respective benzene monocarboxylic acids: benzoic, toluic and dimethyl benzoic acids. Two separate, later and somewhat parallel lower temperature (80.degree. C.-100.degree. C.) modifications of the aldehyde or ketone promoted cobalt catalysis in liquid phase of acetic acid did provide commercially feasible conversion of xylenes to phthalic acids, especially p-xylene to terephthalic acid but only at the expense of using rather high concentrations of cobalt. Combinations of cobalt and manganese with a source of bromine became preferred for commercial use and are disclosed in U.S. Pat. No. 2,833,816. However, cobalt is very expensive and also available only from sources outside the United States and from countries which may cut off the supply of this valuable metal.
For liquid-phase oxidation of dimethylbenzenes or pseudocumene with molecular oxygen it has been discovered that nickel and zirconium are unique among metals for substantially enhancing the activity of the bromine-manganese systems of catalysis.
It is surprising that nickel and zirconium are effective in combination with manganese. In our process it is critical that bromine be present. Good yields are not obtained if bromine is not present. Japanese Kokai No. 77 10,229, German Offenlegungsschrift No. 2,804,156, U.S. Pat. No. 2,833,816 and European patent application No. 0,041,784 disclose nickel catalysts, but not in combination with manganese or zirconium.
The world capacity exceeds ten billion pounds of terephthalic acid. Presently, purified terephthalic acid is produced in two stages, (1) oxidation of paraxylene using dioxygen, a cobalt/manganese/bromine catalyst in an acetic acid solvent, and (2) purifying the crude TA cake by recrystallizing and hydrogenating it in water. Cobalt is the most expensive component of the catalyst system. Therefore, there is great economic incentive to replace cobalt with some other metal. Our novel process has succeeded in doing just this.
For the present invention the gram-atom ratio of nickel-zirconium-manganese is in the range of about 33:1:12 to about 80:1:43. The preferred range is about 40:1:20 to about 70:1:40. The ratio of total metals, Ni plus manganese plus zirconium to bromine is in the range of about 0.5 to about 1.5 on the milligram atom basis. Thus for each gram-mole of p-xylene, m-xylene, or pseudocumene in the oxidation there is used from about 4 to about 20 milligram atom nickel, about 0.10 to about 0.30 milligram atom zirconium, about 2 to about 10 total of Mn and from about 8 to about 24 milligram atoms bromine. The preferred metal to bromine ratio for pseudocumene is about 0.7 to about 1.0 and for metaxylene it is about 0.7 to about 1.0 and for paraxylene it is about 0.7 to about 1.0. The nickel-zirconium catalyst is also useful in oxidation system process, where water was replaced with aliphatic acids containing less than five carbon atoms, such as acetic acid as the reaction medium.
Nickel and zirconium can be added to the reaction in any form soluble in the di- or trimethylbenzene being oxidized in acetic acid. For example, nickel or zirconium octanoate or naphthanate can be used with manganese octanoates or naphthenics for oxidation of the di- or trimethylbenzenes in the absence of reaction solvent and Ni, Zr and Mn can be conveniently used as their acetates when di- or trimethylbenzenes are oxidized in the presence of acetic acid solvent. Nickel and zirconium are readily available and are ideally suited for liquid-phase oxidations using acetic acid or water as reaction solvent.
The source of molecular oxygen for the nickel and zirconium enhanced oxidation of this invention can vary in O.sub.2 content from that of air to oxygen gas. Air was the preferred source of molecular oxygen for oxidations conducted at temperatures at 100.degree. C. and above up to 260.degree. C. For oxidations conducted with molecular oxygen the preferred temperatures were in the range of about 120.degree. C. to about 220.degree. C. The minimum pressure for such oxidations was that pressure which will maintain a substantial liquid phase 70-80%, of the reaction medium either neat di- or trimethylbenzene or such methylbenzene and 70-80% of the acetic acid. The acetic acid or water solvent, when used, can amount to 1-10 parts on weight basis per part of the di- or trimethylbenzene. The methylbenzene and/or acetic acid not in the liquid phase because of vaporization by heat of reaction was advantageously condensed and the condensate returned to the oxidation as a known means for removing heat and thereby temperature controlling the exothermic oxidation reaction.
The benefits to be derived from the use of nickel and zirconium according to the present invention were indicated by results shown with respect to the following illustrative and comparative oxidations using pseudocumene, m-xylene or p-xylene as the methyl-substituted benzene to be oxidized.
The batchwise oxidations were conducted by charging all of the catalyst components, pseudocumene, p-xylene or m-xylene and acetic acid or water, sealing the reactor; setting a pressure control valve initially to 150 psig (valve was in exhaust vent line); pressuring the reactor to 150 psig with nitrogen; heating the reactor contents to the desired temperature, 160.degree. C. for pseudocumene, and then introducing pressurized air into the liquid phase in the reactor at a constant flow rate. Cooling water at approximately 20.degree. C. was introduced into the jacket of the condenser section. Each oxidation was terminated as close to 14 percent vent oxygen by volume as was feasible to do.
In the examples to follow all oxidations were conducted initially at a gauge pressure of 150 pounds per square inch (psig) and at oxidation initiation temperature of 160.degree. C. for pseudocumene, using a weight ratio of acetic acid to pseudocumene or xylene of about 1.87:1 and using air as the source of molecular oxygen. The oxidation reactor used was a stirred 2-liter titanium cylindrical autoclave. A water-cooled condensor was placed immediately above the autoclave to condense and return a substantial portion of the volatile compound. Following the condensation system, there were means for venting the exhaust gaseous mixture (nitrogen, unused or excess oxygen, oxides of carbon, water vapor, and vapor of uncondensed acetic acid and some of the unreacted xylene) and analytical means for determining the oxygen, carbon dioxide, and carbon monoxide contents of exhaust sample on acetic acid-free dry basis. The exhaust sample flowed through three cooled (e.g. dry ice-acetone cooled) traps before analysis for O.sub.2, CO.sub.2 and CO. The reactor was charged with 225 grams of pseudocumene or xylene, thus 420 grams acetic acid for the 1.87:1 solvent to pseudocumene weight ratio. The oxidation of pseudocumene was conducted batchwise by charging all of the catalyst components, pseudocumene and acetic acid, to the reactor. The reactor was sealed. The pressure control valve was set initially at 150 psig (valve was in exhaust vent line). The pressure was gradually increased from 150 psig to 400 psig for 55 minutes. Accordingly the temperature increased from 160.degree. C. to 210.degree. C. in the same time period. The reactor was pressured to 150 psig with nitrogen and then heated to the initiation temperature. Thereafter pressurized air was introduced into the liquid phase in the reactor. Each oxidation was terminated as close to 14% vent oxygen by volume as was feasible to do.
After termination of the oxidations, the total reactor effluents (hereafter "TRE") was collected. The resulting TRE products were submitted for aromatic acid analysis.
Product yield were calculated (and hereafter reported) in mole percent of product per mole pseudocumene or xylene charged.